The present invention relates to a modified cellulosic, lignocellulose or polysaccharide having superabsorbent properties and, more particularly, to a sulfated cellulosic, lignocellulose or polysaccharide.
Personal care absorbent products, such as infant diapers, adult incontinent pads, and feminine care products, typically contain an absorbent core that includes superabsorbent polymer particles distributed within a fibrous matrix. Superabsorbents are water-swellable, generally water-insoluble absorbent materials having a high absorbent capacity for body fluids. The superabsorbent polymers (SAP""s) in common use are mostly derived from acrylic acid, which is itself derived from oil, a non-renewable raw material. Acrylic acid polymers and SAP""s are generally recognized as not being biodegradable. Despite their wide use, some segments of the absorbent products market are concerned about the use of non-renewable oil derived materials and their non-biodegradable nature. Acrylic acid based polymers also comprise a meaningful portion of the cost structure of diapers and incontinent pads. Users of SAP are interested in lower cost SAP""s. The high cost derives in part from the cost structure for the manufacture of acrylic acid which, in turn, depends upon the fluctuating price of oil. Also, when diapers are discarded after use they normally contain considerably less than their maximum or theoretical content of body fluids. In other words, in terms of their fluid holding capacity, they are xe2x80x9cover-designedxe2x80x9d. This xe2x80x9cover-designxe2x80x9d constitutes an inefficiency in the use of SAP. The inefficiency results in part from the fact that SAP""s are designed to have high gel strength (as demonstrated by high absorbency under load or AUL). The high gel strength (upon swelling) of currently used SAP particles helps them to retain a lot of void space between particles, which is helpful for rapid fluid uptake. However, this high xe2x80x9cvoid volumexe2x80x9d simultaneously results in there being a lot of interstitial (between particle) liquid in the product in the saturated state. When there is a lot of interstitial liquid the xe2x80x9crewetxe2x80x9d value or xe2x80x9cwet feelingxe2x80x9d of an absorbent product is compromised.
In personal care absorbent products, U.S. southern pine fluff pulp is commonly used in conjunction with the SAP. This fluff is recognized worldwide as the preferred fiber for absorbent products. The preference is based on the fluff pulp""s advantageous high fiber length (about 2.8 mm) and its relative ease of processing from a wetlaid pulp sheet to an airlaid web. Fluff pulp is also made from renewable, and biodegradable cellulose pulp fibers. Compared to SAP, these fibers are inexpensive on a per mass basis but tend to be more expensive on a per unit of liquid held basis. These fluff pulp fibers mostly absorb within the interstices between fibers. For this reason, a fibrous matrix readily releases acquired liquid on application of pressure. The tendency to release acquired liquid can result in significant skin wetness during use of an absorbent product that includes a core formed exclusively from cellulosic fibers. Such products also tend to leak acquired liquid because liquid is not effectively retained in such a fibrous absorbent core.
A need therefore exists for a superabsorbent material that is simultaneously made from a biodegradable renewable resource like cellulose or other lignocellulosic or polysaccharide, that is inexpensive, and that has a low void volume when saturated. In this way the superabsorber can be used in absorbent product designs that are efficient such that they can be used closer to their theoretical capacity without feeling wet to the wearer. These and other objects are accomplished by the invention set forth below.
In one aspect, the present invention provides a superabsorbent polymer. The polymer of the invention is a modified cellulose, modified lignocellulose, or modified polysaccharide having superabsorbent properties. The modified polymer is a sulfated polymer. In one embodiment the polymer is a sulfated cellulose; in another embodiment, a sulfated lignocellulose; and in another embodiment, a sulfated polysaccharide. The polymer of the invention is a water-swellable, water-insoluble polymer having a high liquid absorption capacity and a low free liquid value. In one embodiment, the polymer has a liquid absorption capacity greater than about 20 g/g. In another embodiment, the polymer has a free liquid value of less than about 40 percent by weight based on the total amount of liquid absorbed. In a further embodiment, the polymer has a free liquid value of less than about 30 percent by weight based on the total amount of liquid absorbed. In one embodiment, the modified polymer is a crosslinked polymer.
In other aspects of the invention, methods for making the superabsorbent polymer and absorbent products that incorporate the superabsorbent polymer are provided.